The invention relates to a method for the reproduction of an audio signal recorded according to the helical scan method. The invention furthermore relates to a recorder for the reproduction of an audio signal recorded according to the helical method. In particular, the invention is based on a video recorder with improved signal recording.
The improved audio signal recording e.g. in accordance with the VHS standard is based on the recording of two sound carriers, which are frequency-modulated with the respective audio signal and called FM audio signal in the following text, in the slanted tracks of the recording medium in tape form.
A video recorder of this type is disclosed e.g. by EP-0-068-188 B1. The respective changeover, necessary in the reproduction mode, between the signal paths of the two read/write heads arranged on a head drum, which are also called channel I and II, is effected in that document in each case in the path of the demodulated FM audio signals at a point in time at which both read/write heads supply an uninterrupted sound carrier on account of partially overlapping recording, two FM demodulators being used for each rotating read/write head for the purpose of demodulation, with the result that the FM demodulators of the two read/write heads can output an uninterrupted audio signal in the overlap region.
For the demodulation of FM audio signals in contemporary video recorders with improved audio signal recording, voltage-controlled oscillators each with a PLL circuit are usually used. For this purpose, the respective PLL circuit forming a phase-locked loop receives, on the one hand, the FM audio signals scanned by the read/write head assigned to it and, on the other hand, the signal from its voltage-controlled oscillator, called VCO in the following text, which is in this case synchronized with the FM audio signal corresponding to its frequency in that the regulating voltage which is produced at the output of the PLL circuit and simultaneously forms the demodulated audio signal controls the VCO.
An object of the invention is to specify an improved method for the interaction of such FM demodulators, used in particular for improved audio signal recording, with the rotating read/write heads.
The object is achieved by means of the method disclosed in this application. Advantageous developments of the invention are specified in the respective subclaims.
A further object of the invention is to provide a recorder in which such FM demodulators, used in particular for improved audio signal recording, interact better with the rotating read/write heads.
This object is achieved by means of the recorder disclosed in this application. Advantageous developments of the invention are specified in the respective subclaims.
The invention is based on the following insights and considerations:
Whenever one of the two rotating read/write heads in the reproduction mode leaves the wrap-around region of the recording medium in tape form on the periphery of the head drum, the FM demodulators assigned to this read/write head do not receive a signal until the said head re-enters the recording medium (that is to say until the said head re-engages with the recording medium), with the result that the respective phase-locked loop is interrupted in each case for a corresponding period. FM demodulators of this type generally operate satisfactorily in video recorders with improved audio signal recording. However, given such an interruption, there is the risk of the frequency of the VCO of the respective FM demodulator drifting and only being able to be regulated to the desired value and/or synchronized with the FM audio signal assigned to it when the corresponding read/write head re-enters the recording medium.
The idea of the invention, therefore, is for the FM demodulators of the respective read/write head which is currently not located in the wrap-around region to be fed the respective signal scanned by the other read/write head, with the result that the period of interruption of the respective phase-locked loop is shortened.
This has the advantage that the VCOs used in particular for the purpose of demodulating FM audio signals in a video recorder can be synchronized in good time with the corresponding FM audio signals.
According to the invention, provision may also be made for feeding to the respective FM demodulators of the read/write head (or to the respective FM demodulator of the read/write head in the case of a recorder in which the respective changeover between the signal paths of the two read/write heads is performed prior to the demodulation of the FM audio signals) which is not currently located in the wrap-around region, in each case for the entire duration of its movement outside the wrap-around region, the FM audio signal which is scanned by the other read/write head, with the result that an interruption of the phase-locked loops practically never occurs.
In an advantageous manner, the FM demodulators can be utilized as modulators for improved recording of the audio signal.